Recently it has been required in a radio communication system, a radar system and the like that an antenna should be decreased in size and thickness according to miniaturization of electronic circuits.
Since the aperture area of the antenna almost depends upon the frequency and gain of the antenna required in the system, it is important that the antenna should be thinned to decrease the volume of the whole antenna.
Conventionally, in order to attain the above object, a microstrip array antenna and a waveguide slot array antenna have been put to practical use as a typical thin planar antenna.
In the microstrip array antenna, a microstrip is formed on a substrate and employed as an antenna element. Since the antenna element can be manufactured by printing technique, the microstrip array antenna is relatively easy to manufacture.
The microstrip array antenna has a drawback in which a frequency band is narrow and a transmission loss of a feeder in an millimeter wave band is considerably larger than that in a microwave band.
The microstrip array antenna is therefore applied to only an array constituted of a few elements and it is not suitable for a system requiring a high gain antenna such as high-speed-and-large-capacity communications and high-resolution sensing in which the use of millimeter waves is expected.
On the other hand, the waveguide slot array antenna includes a waveguide having a slot as an antenna element. For example, a waveguide slot array antenna as described in Jpn. U.M. Appln. KOKOKU Publication No. 7-44091 is known in which a plurality of radiating waveguides are so arranged that one end portion of each radiating waveguide is hit against the side of a feeding waveguide to feed power from the feeding waveguide to each of the radiating waveguides.
Such a waveguide slot array antenna decreases in transmission loss in a high-frequency band such as submillimeter and millimeter wave bands and is therefore suitable for a system that necessitates a high-gain antenna.
In the waveguide slot array antenna, however, the feeding waveguide and the plurality of radiating waveguides are generally formed by vertically fixing a side wall for the feeding waveguide and the radiating waveguides on a common base and fixing a slot plate for the plurality of radiating waveguides thereon.
For this reason, the waveguide slot array antenna so constituted necessitates a manufacture process such as welding in order to complete electrical contact between the upper edges of side walls of the waveguides and the slot plate, and has problems in which its productivity is low and its price is difficult to lower.
In order to resolve the structural problems of the waveguide slot array antenna, there is proposed a method for feeding power to adjacent waveguides in opposite phases to make the side walls of the waveguides and the surface of the slot not contact with each other.
The above method, however, had a problem in which the waveguides were easily joined with each other and the antenna characteristics were degraded.
Further, an antenna used for a car-mounted radar is not only small but also requires a beam scan in order to detect an obstacle with high resolution and prevent an error in detection due to a difference between the direction of the body of a car running on a curve and that of the running car.
To meet the above requirements, conventionally, a method for scanning with a beam by mechanically moving a radar antenna has been employed.
Such a mechanical beam scanning method has drawbacks in which a radar apparatus is increased in size for a driving mechanism and decreased in reliability.
It is thus desired that an electronic beam scanning method be put to practical use in place of mechanical beam scanning.
As an electronic beam scanning method, there are a method for switching a plurality of antennas having different beam directions by means of a switch and a so-called phased array antenna for varying a phase of feeding to a plurality of antennas by a variable phase shifter and then varying a direction of a synthesized beam.
Since the former method makes use of only some of the plurality of antennas, there occurs a problem in which the whole antenna is increased in size in order to obtain a narrow beam and a high gain.
The latter method has a problem in which beams need to be synthesized using a variable phase shifter for each antenna and thus the antenna is complicated in structure and increased in cost.